Wireless or Ethernet over power or....

[#1]

From your description I believe that the builders terminated the wires in the extensions and probably left the cable at the master socket. When BT Openreach then came to connect the telephone up the Engineer then terminated this cable to the faceplate. If this is the case there is a possibility that the wiring is not correct. Check the colour of the wires on the faceplate in terminals 2 and 5 and then check the extensions is see if the same colours are being used.

[#2]

Okily, I took off the faceplate again and managed to grab a photo with my phone.  2 is red, 3 is blue, 5 is white.  It's upside down in the picture
/edit - just reading up on NT5's and that little round thing just above my finger is apparently a "bell circuit filter", I'm going to have to read up on that!  Does that filter make any difference?
/edit2 - reading on this filter, it seems it is sort of like the new "iplate" filter thingymabob?
/edit3 - Ok, reading more it seems that with this filter built in, you cannot plug ADSL into anywhere other than the master socket?  Is that correct? O_o
/edit4 - itsme, you are (I think) totally correct!  I took out the socket in my room and took a picture.  2 is blue, 3 is orange (not red as I previously stated) and 5 is white!  That and Kitz's site proved invaluable!  So who's responsible for switching them around (new build house)?  Or is it an easy task that someone who thinks a hammer is good for knocking a dent out of glass is a good idea?
Pic DSC00071 isn't very clear, its the thingy in my room.

[#3]

The builders should put it right - under the NHBC guarantee 
http://www.nhbc.co.uk/Homeowners/Problemswithyourhome/Whatiscovered/FAQ/#d.en.34761
Don't you have a snagging list on all faults in the house ?

[#4]

It seem the the cable used is external drop wire. Not sure whether this type of cable is readily available outside BT.
As for getting done under the NHBC guarantee, who made the mistake? Did the builders terminate the cable at the master socket? BT would normally use the Orange and White as the first A and B pair so it suggest that the master was terminated by BT and the extensions by the builders.
As regarding edit 3/ the iplate is just a filter for the ring wire it's not an ADSL filter so any socket can be used for the modem/router  and ADSL filters will need to be used on each extension that is used..

[#5]

Quote

It seem the the cable used is external drop wire.

Hang on a minute , didnt the OP say earlier on that the phone worked from the test socket ? If it does then the incoming drop can't possibly be wired to the removeable faceplate ! Maybe the builders just used whatever cable they had ( possibly chatted up a BT engineer who was on site ? _

[#6]

I'll let the housing manager person know (i.e. bug her till she agrees to get it fixed) on Monday!  Though as its not an "emergency" the timescale for getting it sorted is up to 6 months, just to flip a couple of wires
/update - decided to try it myself rather than wait 6 months, switched the wires around and gently put them in to the slots with a thin flathead, works fine and my line attenuation went from 31db down to 30db